1. Field of the Invention
The present invention relates to a device for generating an AC ramp for checking the tripping current of a circuit-breaker, comprising a bridge rectifier whose terminals on the AC side are connected respectively to an active terminal of the AC voltage network, situated downstream of the circuit-breaker, and to the system ground or ground or another active terminal of the network situated upstream of the circuit-breaker, a load circuit connected between the two terminals of the bridge rectifier on the DC side, this load circuit being formed principally by a power transistor in series with a measuring resistor, the control terminal of this power transistor being connected to the output terminal of a high gain amplifier, whose first input receives at least a part of the voltage appearing at the terminals of the measuring resistor, a field effect transistor drive by a voltage depending on the voltage at the terminals of a capacitor charged from an auxiliary battery, the conducting resistance of this field effect transistor forming, with a resistor, a potentiometric divider whose middle point is connected to a second input of the high gain amplifier, the potentiometric divider being connected between the two terminals on the DC side of the bridge rectifier.
2. Description of the Prior Art
AC current ramp generating devices are well known (see for example French Pat. No. 2 377 089). They are used for checking the tripping current of a circuit-breaker more especially a residual differential current circuit-breaker, whose protection device is responsive to a current difference, thus allowing a fault current to the system ground or to ground to be detected. In circuit-breakers of this kind, the device for protecting the circuit-breaker must cause the circuit-breaker to trip, i.e. breaking of the current, for a current less than or equal to a current I.sub..DELTA.n, called nominal operating current, and higher than a nominal non operating current, whose value is generally equal to half the value of the nominal operating current. The value of the nominal operating current and the value of the nominal non operating current are defined by standards and, in service, it is necessary to check from time to time the value of the current for tripping the circuit-breaker so as to make sure that it remains between the above mentioned values. This check may be carried out by means of an AC current ramp generating device which produces an increasing AC current following a ramp from a minimum predetermined value, which is less than the nominal non operating current, to another predetermined value which is greater than the nominal operating current.
For the measures carried out at each check to be significant, it is desirable for the AC current ramp generated by the generating device to be as linear and as stable in time as possible. Now, in the known AC current ramp generating device (French Pat. No. 2 377 089), the definition and linearity of the AC current ramp are dependent:
(a) on the value and on the stability of the AC voltage of the network supplying the above mentioned potentiometric divider; PA0 (b) on the pseudo-linearity of growth in time of the conduction resistance of the field effect transistor from the exponential development of the voltage at the terminals of the capacitor which is charged from the auxiliary battery; PA0 (c) on the value of the voltage of the auxiliary battery, which is directly involved in the rate of increase of the voltage at the terminals of said capacitor.